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ARS Home » Southeast Area » Griffin, Georgia » Plant Genetic Resources Conservation Unit » Research » Research Project #442480

Research Project: Estimating Biological Nitrogen Fixation of the USDA Cowpea Germplasm

Location: Plant Genetic Resources Conservation Unit

Project Number: 6046-21000-013-033-S
Project Type: Non-Assistance Cooperative Agreement

Start Date: Aug 1, 2022
End Date: Dec 15, 2023

The ultimate goal of this proposal is to identify cowpea germplasm with effective biological nitrogen fixation (BNF). The specific objectives are: Objective 1. To estimate biological nitrogen fixation (BNF) of the USDA germplasm under two water management regimes (dryland and full irrigation). Objective 2. To establish the relationship between BNF, biomass and seed yield of USDA cowpea germplasm.

Objective 1. Plant materials: A total of 250 United States Department of Agriculture (USDA) cowpea accessions obtained from the Plant Genetic Resources Conservation Unit, Griffin, Georgia, will be used for this project. This experiment will be conducted at Texas A&M AgriLife Research- Vernon. Two water regime treatments will be applied. The first water regime treatment will be dryland (rainfed) to reflect real farmers’ conditions. The second water regime will be fully irrigated (furrow irrigation), and irrigations will be conducted before planting and in two-week interval until pod filling is established. Commercial certified inoculants (powder) will be added to seeds prior to planting to promote nitrogen fixation. Fertilizers (P and K) will be applied based on soil test results. However, no nitrogen (N) fertilizers will be applied. Experiment designs: The study will be laid out in a randomized complete block design with two blocks (replications) for each water regime. The experimental unit will be defined by a two-row plot of 17 feet long (15 feet for cowpea planting and 2 feet alley) and 3 feet spacing between rows. Plant spacing within each row will be 4 inches. Three representative plants will be collected from each plot and combined to estimate the rate of BNF by the natural abundance method. Leaves from five randomly selected plants will be harvested and dried, then ground to pass a 1-mm sieve. Ground samples will be analyzed for total N and delta 15N isotope contents at the Texas A&M Stable Isotopes for Biosphere Science Laboratory. During BNF, the heavier stable isotope, d15N, is discriminated against by bacteria and its subsequent transfer to the plant is less than delta 14N relative to a non-N-fixing reference plant. In this case, the reference plant will be cotton because it is a widely grown non-N-fixing summer crop in our region. The delta 15N values that result from isotope analysis will be used to provide an estimate of percent N derived from the atmosphere (%Ndfa) by cowpea in each plot. For each cowpea line, observation of root nodules will be made at the flowering stage, by digging up a 30x30x30 cm area of soil surrounding one representative plant in each plot. The soil will be placed in bins, then hand sifted to remove nodules. The nodules will be counted as they are removed from the soil, then washed with water. Clean nodules will be split open to make observations of the color of the interior tissue, with red interior signifying activity and grey/blue or other off-colors signifying inactivity. Objective 2: For each cowpea line, a representative three-foot section from one row will be used for plant biomass estimation. Biomass will be collected when most plants are at the first bloom stage and the plant biomass will be oven-dried at 60°C. The samples will be daily weighed, and the dry biomass weight will be recorded when weight is constant. At harvest, pods from the remaining plants will be hand-harvested, pods threshed and seeds weighed to calculate seed yield.